1. Field of the Invention
The present invention relates to apparatus (devices) for the transmission of information by a laser beam and laser oscillators therefore. This apparatus includes at least one laser oscillator for generating a laser beam and means for varying a characteristical quantity of the laser beam. The laser may take the form of a surface-emitting semiconductor laser, in which case the variable quantity is the spatial mode content of the laser beam. The laser may be provided with electrically variable means for extracting wave energy from a laser wave in the resonator of the laser, in which case the varying quantity is the spatial mode content or the frequency of the laser beam.
2. Description of the Prior Art
An information transmission system is the ensemble of components, which contribute to the transmission of information from an emitter to a receiver. The information is transmitted by the modulation of a characteristical quantity of the carrier wave by a signal wave, which has to be transmitted by means of this carrier wave from an emitter to one or several receivers. The modulation may be created by variations of the intensity of the carrier wave or of the phase angle of the carrier wave. For example, if the intensity of the carrier wave is emitted by pulses, which are controlled in some aspect by the signal wave, pulse modulation takes place. If the frequency of the carrier wave follows in some aspect the signal wave, frequency modulation takes place. If the phase change of the carrier wave obeys in some aspect the signal wave, phase modulation takes place.
A carrier wave and particularly a laser beam is characterized by its intensity and phase angle, as described above, and by its spatial mode content. The spatial mode content of a laser beam is characterized for example by the field distribution in a plane perpendicular to the directions of propagation of the beam, which means that the spatial mode content of a laser beam is characterized by the curvature of a phase front of the laser beam and by the spatial intensity distribution in the plane of such a phase front.
A large variety of oscillators with means for the variation of the oscillator wave are known, particularly optical lasers. Typical of this large body of prior art are the following references:
(1) Y. Arakawa et al., "Active Q switching in a GaAs/AlGaAs multiquantum well laser with an intracavity monolythic loss modulator", Appl. Phys. Lett. 48, pp. 561-563, 1986. PA0 (2) F. K. Reinhart et al., "Integrated electro-optic intracavity frequency modulation of double-heterostructure injection laser", Appl. Phys. Lett. 27, pp. 532-534, 1975. PA0 (3) F. K. Reinhart et al., "Electro-optic frequency- and polarization-modulated injection laser", Appl. Phys. Lett. 36, pp. 954-957, 1980. PA0 (4) S. Uchiyama et al., "GaInAsP/InP Surface-Emitting Lasers with Current Confining Structure", J. Lightwave Technol. LT-4, pp. 846-851, 1986. PA0 (5) Z. L. Liau et al., "Surface-emitting GaInAsP/InP laser with low threshold current and high efficiency", Appl. Phys Lett. 46, pp. 115-117, 1985. PA0 (6) R. Schimpe, "Cylindrical diffraction grating couplers and distributed feedback resonators for guided wave devices", U.S. patent application Ser. No. 06/814,612, 1985.
In Ref. 1, the arrangement of externally controllable absorption structures in optical laser resonators is used for switching of the intensity of a laser wave generated in the resonator. In Ref. 2, the change of the refractive index due to an electric field is used for frequency modulation. In Ref. 3, the wavelength shift of the maximum of the optical amplification over several longitudinal modes is used for tuning of the laser wavelength. In the Refs. 4 and 5, surface-emitting semiconductor lasers are described, which are characterized by the emission of the laser output beam essentially perpendicular to the substrate on which an active region of the semiconductor laser is grown. In Ref. 6, a surface-emitting semiconductor laser generating a laser beam with definite mode content is described by providing means for the selection of the mode content of this laser beam.
It is not known, however, to arrange electrically variable means for extracting energy from a laser wave within the resonator of a laser, with the aim to generate a laser beam with varying spatial mode content, whereas the intensity and the frequency of the laser beam are essentially constant (hereafter referred to as "mode modulation"). It is not known to operate an apparatus of the above mentioned type with the aim to generate a laser beam with varying frequency, whereas the intensity and the spatial mode content of the laser beam are essentially constant. It is also not known to provide a surface-emitting semiconductor laser with electrically variable means for selecting the spatial mode content of the laser beam. Furthermore, it is not known to have an information transmission system, wherein the information is transmitted essentially by a modulation of the spatial mode content of a laser beam.